1. Field of the Invention
This invention relates to an ion source which has a filament for emitting electrons and a reflector for reflecting the electrons and which applies a magnetic field to the inside of a plasma production vessel, and more particularly to means for improving an ion production efficiency, prolonging the life of the filament, etc.
2. Description of the Related Art
FIG. 12 shows a related art example of the ion source. This ion source is called Bernas-type ion source. An ion source of a similar structure is also disclosed, for example, in Japanese Patent Unexamined Publication No. Hei. 9-63981.
The ion source comprises a plasma production vessel 2, for example, shaped like a rectangular parallelepiped and also serving as a positive potential. Gas (containing also the case where the gas is vapor) for producing plasma 16 is introduced into the inside of the plasma production vessel 2. The plasma production vessel 2 is formed on a wall face (long-side wall) with an ion extraction slit 4 for extracting an ion beam 18. In the example, the ion beam 18 is extracted toward the rear of the plane of the figure.
A filament 6, for example, shaped like U, for emitting an electron e is placed in one side (one short-side wall side) of the plasma production vessel 2. The filament 6 and the plasma production vessel 2 are electrically insulated by an insulator 12.
An opposed reflector 8 for reflecting the electron e is placed facing the filament 6 in an opposite side of the plasma production vessel 2 (namely, the other short-side wall side facing the filament 6). The opposed reflector 8 and the plasma production vessel 2 are electrically insulated by an insulator 13. The opposed reflector 8 may be placed in a floating potential without connecting to any point. The opposed reflector 8 may be also connected to one end of the filament 6 (more particularly, the negative potential terminal of a filament power supply 24) by a conductor 28 for placing the opposed reflector 8 in filament potential as described in the above-mentioned Japanese Patent Unexamined Publication No. Hei 9-63981.
A rear reflector 10 for reflecting the electron e is placed facing the opposed reflector 8 at a place positioned behind the filament 6 in the plasma production vessel 2. Namely, the rear reflector 10 is placed between the U-shaped portion of the filament 6 and the wall face of the plasma production vessel 2 behind the U-shaped portion. The rear reflector 10 and the plasma production vessel 2 are electrically insulated by insulators 12 and 14. The rear reflector 10 has been connected to one end of the filament 6 (more particularly, the negative potential terminal of the filament power supply 24) for placing the rear reflector 10 in filament potential.
In the plasma production vessel 2, a magnetic field generator 20 placed outside the plasma production vessel 2 applies a magnetic field 22 along the axis connecting the filament 6 and the opposed reflector 8 to produce and confine the plasma 16. However, the direction of the magnetic field 22 may be opposite to that shown in the figure. The magnetic field generator 20 is, for example, an electromagnet.
DC filament voltage VB (for example, about 2 to 4 V) is applied from the DC filament power supply 24 to the filament 6 to heat the filament 6 for emitting an electron (thermoelectron) e.
From a DC arc power supply 26, arc voltage VA (for example, about 40 to 100 V) is applied between one end of the filament 6 and the plasma production vessel 2 with the filament 6 as the negative potential to produce arc discharge between the filament 6 and the plasma production vessel 2.
FIG. 13 shows an example of potential variation in the ion source according to the elated art. In the example, the opposed reflector 8 is connected to one end of the filament 6 by the conductor 28. However, if the opposed reflector 8 is not connected to any point for placing the opposed reflector 8 in floating potential, the potential of the opposed reflector 8 becomes the same extent as that in the example, namely, the same extent as the potential of the filament 6. The reason is that if the opposed reflector a is placed in the floating potential, a far larger number of light and high-mobility electrons in the plasma 16 than the number of ions are incident on the opposed reflector 8 and thus the opposed reflector 8 is charged at negative potential.
The gas introduced into the inside of the plasma production vessel 2 is ionized by the above-mentioned arc discharge to produce the plasma 16. From the plasma 16, the ion beam 18 can be extracted by an electric field. Usually, an extraction electrode for extracting the on beam 18 is placed at a point opposed to the ion extraction slit 4 (the rear of the plane of the figure), but is not shown here.
The production process of the plasma 16 will be discussed in detail. The electron e emitted from the filament 6 is accelerated toward the plasma production vessel 2 by the above-mentioned arc voltage VA (the filament voltage VF is small as mentioned above and therefore is ignored in the description). Then accelerated electron e with the energy corresponding to the voltage VA collides with a gas molecule for ionizing the gas molecule, whereby plasma 16 is produced. The ions and electrons (also containing thermoelectrons emitted from the filament 6) e in the plasma 16 are trapped by the above-mentioned magnetic field 22 and further repeat collision with gas molecules, thereby producing and confining the plasma 16.
The potential of the plasma 16 becomes a potential between the potential of the plasma production vessel 2 and the potentials of both the reflectors 8 and 10, as shown in FIG. 13, and a potential difference occurs between the plasma 16 and both the reflectors 8 and 10. The potential difference causes electrons e emitted from the filament 6 or produced in the plasma 16 to be reflected on both the reflectors 8 and 10 and reciprocate between both the reflectors 8 and 10. Consequently, the collision probability between the electrons e and gas molecules is increased and plasma 16 with a high density can be produced. As a result, the extracted ion beam 18 can be increased.
There is a demand for extracted multiply charged ions of doubly charged or more ions for use as the ions forming the ion beam 18 from the ion source as described above. The reason why there is such a demand is that a multiply charged ion can provide acceleration energy valence times that of a singly charged ion at the same acceleration voltage (for example, a doubly charged ion provides acceleration energy twice that of a singly charged ion) and thus high energy can be easily provided.
However, in the ion source in the related art as described above, production of multiply charged ions is not considered and thus the production amount of the multiply charged ions is small as compared with that of molecular ions or singly charged ions. That is, the ratio of the multiply charged ions in the plasma 16 and thus the ratio of the multiply charged ions contained in the ion beam 18 are not high. Therefore, the multiply charged ions cannot be used effectively.
An object of the present invention is to provide an ion source and operation method thereof which can improve the production efficiency of multiply charged ions in an ion source for increasing the ratio of multiply charged ions contained in an ion beam. Other objects are described later.
In order to accomplish the object above, the following means are adopted. According to the present invention, there is provided an ion source of a first aspect comprising a rear reflector, an opposed reflector, a filament, a filament power supply, a plasma production vessel, an arc power supply, and a DC bias power supply. The rear reflector is electrically insulated from the filament and the plasma production vessel. The DC bias power supply is a power supply individuated from the filament power supply and the arc power supply. The DC bias power supply is provided for applying a DC bias voltage between at least one of the opposed reflector and the rear reflector and the plasma production vessel with the reflector as a negative potential.
In the ion source, the potential of at least one of the opposed reflector and the rear reflector can be adjusted based on the bias voltage applied from the bias power supply independently of the output voltages of the arc power supply and the filament power supply. Therefore, the energy and the amount of the electrons reflected on the reflector can be adjusted according to the bias voltage. For example, the energy and the amount of the electron which reflected are increased with increasing the bias voltage.
In the ion source, it is possible to use many high-energy electrons to produce plasma and thus it is possible to more increase ionization of molecules, atoms, or ions in the plasma and produce a larger number of multiply charged ions. That is, it is possible to improve the production efficiency of multiply charged ions for increasing the ratio of the multiply charged ions contained in the ion beam.
In case of singly charged ion beam extraction, many high-energy electrons reflected on the reflector to which the bias voltage is applied can also be efficiently used to produce the plasma for enhancing the ion production efficiency, so that it is also possible to improve the singly charged ion production efficiency for increasing the extracted singly charged ion beam.
In the ion source, even if the arc voltage is reduced, the high-energy electrons reflected on the reflector to which the bias voltage is applied can ionize the gas efficiently. Thus, it is possible to prevent reducing the plasma production efficiency and to prevent a decrease in the beam current. Therefore, the filament current and further the arc current need not be made large. Consequently, it is also made possible to reduce the arc voltage for prolonging the life of the filament.
Thus, according to the ion source, if the principal object is to improve the ion production efficiency, the production efficiency of multiply charged and singly charged ions can be enhanced. If the principal object is to prolong the life of the filament, the arc voltage can also be reduced for prolonging the life of the filament. This can be accomplished in singly charged ion production and multiply charged ion production. Both improvement in the ion production efficiency and prolonging the life of the filament can also be intended.
In the ion source, at least one of the opposed reflector and the rear reflector maybe made of a material having a higher thermoelectron radiation current density than tungsten. Thus, it is possible to use also the electrons emitted from the reflector effectively to produce and confine the plasma and thus the filament current required for producing a predetermined arc current can be more reduced. Therefore, it is possible to more prolong the life of the filament.
In the ion source or an operation method thereof, the potential of at least one of the opposed reflector and the rear reflector may be made negative below the potential of the filament as the bias voltage is applied. Further, in the ion source or the operation method thereof, the bias voltage may be set larger 10 V or more than the arc voltage. Therefore, it is possible to use a larger number of high-energy electrons and thus it is possible to more enhancing the effects of improving the ion production efficiency, prolonging the life of the filament, etc., described above.
According to the present invention, there is also provided an ion source of a second aspect comprising first and second rear reflectors, first and second opposed reflectors, a filament, a filament power supply, a plasma production vessel, a arc power supply, and a DC bias power supply. The first and second rear reflectors are electrically insulated from the first and second filaments. The DC bias power supply is a power supply individuated from the filament power supply and the arc power supply. The DC bias power supply applies a DC bias voltage between at least one of the first and second rear reflectors and the plasma production vessel with the reflector as a negative potential.
In the ion source, the potential of at least one of the first and second rear reflectors can be adjusted based on the bias voltage applied from the bias power supply independently of the output voltages of the arc power supply and the filament power supply. Thus, the energy and the amount of the electrons reflected on the reflector can be adjusted according to the bias voltage. Consequently, it is possible to use many high-energy electrons to produce plasma and thus it is possible to more increase ionization of molecules, atoms, or ions in the plasma and improve the ion production efficiency.
Consequently, if the principal object is to improve the ion production efficiency, the production efficiency of multiply charged and singly charged ions can be enhanced. If the principal object is to prolong the life of the filament, the arc voltage car also be reduced for prolonging the life of the filament. This can be accomplished in singly charged ion production and multiply charged ion production. Both improvement in the ion production efficiency and prolonging the life of the filament can also be intended.
Moreover, the ion source has two pairs of filaments and rear reflectors, so that the amount of electrons emitted from each filament can be halved for still more prolonging the life of each filament.
In the ion source, at least one of the first and second rear reflectors may be made of a material having a higher thermoelectron radiation current density than tungsten. Thus, it is possible to use also the electrons emitted from the reflector effectively to produce and confine the plasma and thus the filament current required for producing a predetermined arc current can be more reduced. Therefore, it is possible to more prolong the life of the filament.
In the ion source or an operation method thereof, the potential of at least one of the first and second rear reflectors may be made negative below the potentials of the first and second filaments as the bias voltage is applied. The bias voltage may be set larger 10 V or more than the arc voltage. Therefore, it is possible to use a larger number of high-energy electrons and thus it is possible to more enhancing the effects of improving the ion production efficiency, prolonging the life of the filament, etc., described above.
In the ion source, the plasma can be ignited reliably with a large filament current at the initial condition of operating the ion source and then the filament current may be reduced. By doing this, the life of the filament can be still more prolonged.
Further, in the ion source, the magnitude of the bias voltage output from the bias power supply may be controlled. By doing this, the amount of the ion beam extracted from the ion source can be controlled at high speed as compared with the case where the filament current is changed for changing the arc current.